Dielectric-loaded chokes

ABSTRACT

To provide discrimination between harmonically related signals, the conventional short-circuited and opencircuited transmission line choke is modified by dividing it into two dissimilar regions. In the short-circuited choke, the first region, adjacent to the input end, is loaded by means of a first dielectric material having a first dielectric constant Epsilon 1. The second region, constituting the balance of the line, is loaded by means of a second dielectric material having a second dielectric constant Epsilon 2, where Epsilon 2 is greater than Epsilon 1. In the open-circuited choke, Epsilon 2 is less than Epsilon 1. In either case, the desired input impedance characteristic of the choke is realized by adjusting the magnitudes of Epsilon 1 and Epsilon 2, and the lengths of the two regions.

United States Patent [1 1 [111 3,872,412 Seidel ]Mar. 18, 1975 DIELECTRIC-LOADED CHOKES Primary Examiner-James W. Lawrence 75 Inventor: Harold Seidel, Warren, NJ. i M Nussbaum Attorney, Agent, or FzrmS. Sherman [73] Assignee: Bell Telephone Laboratories, 7 I

Incorporated, Murray Hill, NJ. [57] ABSTRACT [22] Filed: Apr. 26, 1974 To provide discrimination between harmonically related signals, the conventional short-circuited and [2H Appl' 464479 opencircuited transmission line choke is modified by dividing it into two dissimilar regions. in the short- 521 US. Cl. 333/73 R, 333/73 c, 333/76, circuited choke, the first region, adjacent to the input 333/96, 333/97 R end, is loaded by means of a first dielectric material [51] I t, Cl HOl 1/20, HOl 3/06, l-[Ol 7/04 having a first dielectric constant e The second re- [58] Field of Search 333/97 R, 73 W, 76, 73 R, gion, constituting the balance of the line, is loaded by 333/96, 73 C, 73 S, 12, 82 R; 82 B, 83 R means of a second dielectric material having a second dielectric constant 6 where 6 is greater than 6,. In [56] Refe c Cit d the open-circuited choke, s is less than 6,. In either UNITED STATES PATENTS case, the desired input impedance characteristic of the I choke is realized by adjusting the magnitudes of e, and $133128? Z1318 $3333???:iJJiJJJJJJJJ:1:131:313i323fi hhd the hhgths hf the two rhghhh- 12 Claims, 6 Drawing Figures DIELECTRIC-LOADED CHOKES The present invention relates to dielectric-loaded transmission line chokes.

BACKGROUND OF THE INVENTION Many circuits require some means of separating signals of different frequencies. For example, one may wish to block a lower frequency signal while passing a higher frequency signal. A convenient way of realizing this type of frequency discrimination is to insert a quarter-wave, short-circuited stub in the signal path. However, if the higher frequency signal happens to be an odd harmonic of the lower frequency signal, this technique cannot be used as the stub will be an odd multiple of a quarter of a wavelength at both frequencies, and consequently both signals will be blocked equally.

Conversely, one may wish to pass a lower frequency signal while blocking a higher frequency signal. In this latter case, a half-wave, short-circuited stub can be used so long as the two signals are not harmonically related.

Inasmuch as the signals associated with harmonic generators are harmonically related, the so-called quarter-wave and half-wave chokes described hereinabove cannot always be used.

It is, accordingly, the broad object of the present invention to affect frequency discrimination between harmonically related signals using nominal quarterwave and half-wave" chokes.

SUMMARY OF THE INVENTION A choke, in accordance with the present invention, comprises a length of transmission line short-circuited or open-circuited at its end. A first region of the line, adjacent to the input end, is loaded by means of a first dielectric material having a first dielectric constant 6 A second region, constituting the balance of the line, is loaded by means of a second dielectric material having a dielectric constant 6 For the short-circuited choke, 6 is greater than 6 For the open-circuited choke, 6 is less than 6 In either case, the desired input impedance characteristic is realized by adjusting the magnitudes of 6 and 6 and the lengths of the two regions. In one embodiment of a short-circuited choke, the parameters of the two regions are proportioned such that the input impedance of the line is very much higher at a specified frequency than it is at a selected odd harmonic of said specified frequency. In a second embodiment ofa short-circuited choke, the parameters of the two regions are proportioned such that the input impedance of the line is very much lower at a specified frequency than it is at a selected harmonic of said specified frequency, 1

In one embodiment of an open-circuited choke, th parameters of the two regions are proportioned such that the input impedance of the line is very much lower at a specified frequency than it is at a selected odd harmonic of said specified frequency. In a second embodiment of an opencircuited choke, the parameters of the two regions are proportioned such that the input impedance of the line is very much higher at a specified frequency than it is at a selected harmonic of said specified frequency.

These and other objects and advantages, the nature of the present invention and its various features will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a prior art short-circuited transmission line choke; I

FIG. 2 shows the variations of the input impedance of a short-circuited length of transmission line as a function of frequency; FIG. 3 shows a short-circuited transmission line choke in accordance with the invention;

FIG. 4 shows a choke, in accordance with the invention, incorporated into a coaxial cable;

FIG. 5 shows a prior art open-circuited choke; and FIG. 6 shows an open-circuited transmission line choke, in accordance with the present invention.

DETAILED DESCRIPTION Referring to the drawings, FIG. I shows a prior art short-circuited transmission line choke comprising a length of transmission line 10, open-circuited at its input end 11, and short-circuited at its other end 12.

In general, the input impedance Z,-,,of a'length of uniform low-loss transmission line is given by Z,',,=Z [Z cos 6+iZ sin 6 /Z cos 6+iZ sin 6] where Z, is the characteristic impedance of the line: 6 is the electrical length of the line I and Z is the terminating impedance at the end ofthe line.

For a short-circuited choke 2,. O, and the input impedance approaches infinity at a frequencyffor which the line length corresponds to one-quarter of a wavelength. That is, for 6 M4, equation (1) reduces to Z,-,, i Z tan As the frequency is increased. the input impedance decreases to zero at a frequency 2f, and then increases once again, approaching infinity at a frequency 3f. The wellknown variation of the input impedance of a shortcircuited transmission line as a function of frequency is illustrated by the solid line curves in FIG. 2. The important thing to note is that such a length ofline, in its conventional form, will disply the same high input impedance whenever its length is equal to a frequency f for which the line length is equal to a quarter of a wavelength. As such, a quarter-wave choke cannot be used as a means of separating a signal and an odd harmonic of said signal. Similarly, it will be noted that a shortcircuited length of transmission line exhibits a low input impedance at a frequency 2ffor which 6 is equal to one-half a wavelength and at integral multiples thereof. As such, a half-wave choke cannot be used as a means of separating a signal and its harmonics.

What is needed is a choke that will have a high impedance at one frequency of interest and, at the same time, exhibit a much lower impedance at another frequency of interest, where the two frequencies are harmonically related.

In accordance with the present invention, the above described impedance characteristic is realized by the unequal loading of the transmission line. In the case of a short-circuited choke, the short-circuited end of the Thus, a nominal quarter-wave choke, designed in acline is loaded by means of a higher dielectric material cordance with the present invention, has an input imthan the input end of the line. Such a choke, illustrated pedance that is l6 orders of magnitude greater at the in H0. 3, comprises as in the prior art, a length of fundamental frequency than it is at the third harmonic transmission line 20 short-circuited at an end 23. How of the fundamental frequency.

ever, in accordance with the present invention, the line The effect of the dielectric loading upon the input length is divided into two dissimilar regions. The first impedance can be understood by referring once again region, adjacent to the input end 21, is filled with a first to FIG. 2. As noted above, in'the unloaded, quarterdielectric material 22 having a first dielectric constant wave choke the input impedance is a maximum at the q. The second region, adjacent to the short-circuited l0 fundamental frequency f and at all odd harmonics of end 23, is filled with second dielectric material 24 havthe fundamental, as shown by the solid curves. By placing a second dielectric constant where is greater ing a higher dielectric material near the short-circuited than e end of the choke, where the electric field at the funda- The normalized input impedance for such a choke is m n al fr q n y i r l i ly mall, there is only a given by slight perturbation at the fundamental frequency. Howcos no sin n9 sin n6 cos r19 in e C S n9 cos n9 1 sin n9 sin n6 where ever, at the higher frequencies, more of the electric 6, and 0 are the electrical lengths of the two regions .field is crowded into the higher dielectric material, efat the fundamental frequency; and fectively lengthening the choke by 90 degrees. This is n is the order of the harmonic, shown by the broken curves in FIG. 2 which show the There are two possible modes of operation. In a first put impedance peaking at the fundamental freembodiment of the invention, the parameters are seq yif, as before, but P g through a Sefiond lected such that the choke has a high impedance at the maximum at a fr q n y o er than 3f. In this particufundamental frequency and a low impedance at an odd lar illustration, the input impedance is a minimum at harmonic frequency. the third harmonic 3f.

For 2 00 at the fundamental frequency f (Le n in a SeCOl'ld embodiment Of invention, the param- 1 we set the deno i f equation (3) equal to eters are selected such that the choke has a low impedzero Thi gives ance at the fundamental frequency and a high imped- 40 ance at a particular harmonic frequency. COS C05 02 ($52) Sm 01 Sm 92 For Z,-,, 0 at the fundamental (1.e., n l), we set (4) the numerator of equation (3) equal to zero. This gives l/ VeZcos 0, sin 0 l/ \/e ,sin 0 00s 0 0. For Z,-,, O at the harmonic frequency nf, we set the ita at the harmonic eq y. set the numerator of equation (3) equal to zero, thus yielding "Ominaior of equation equal to Zero, thus Obtaining cos n0 cos n0 Va /e sin n0 sin n0 O. 7;

l/ /e cos n6, sin 110 l/ Vs: sin n0 cos n0 0 The two equations (6) and (7) can then be simultaneously solved as explained hereinabove.

H6. 4 shows, in longitudinal cross-section, a section of coaxial cable incorporating a short-circuited choke in accordance with the present invention. The cable comprises an outer, hollow, cylindrical conductor 40 surrounding an inner conductor shown as comprising It will be noted that these two equations have four variables 6,, 0,, c and 0 Accordingly, two parameters can be arbitrarily selected and the other two obtained by the simultaneous solution of the two equations.

EXAMPLE two portions 41a and 41b. The region 42 between the Assume. inner and outer conductors is filled with a dielectric material.

1 1 The end of conductor portion 41a includes a reduced 62 10 diameter region 43 which extends into an adjacent, hol- 6O lowed out end region 45 of conductor portion 41b. By

a fundamental frequency 2 GHZ and making the outside diameter of region 43 less than the n inside diameter of region 45, and the length of region 43 greater than the length of region 45, an annular This gives: short-circuited choke 44 is formed. Signal access is 0, 59.4, or a length of 0.975 inches. through an annular gap 46 which results because of the 0 =-6l.9, or a length of 0.321 inches. unequal lengths of the end regions.

Z at 2 Gl-lz 8.33 X 10 The region of the choke adjacent to gap 46 is filled Z, at 6 GHz 1.16 X 10 vith a first dielectric material having a dielectric con- 3,872,412 5 6 stant 6,. The short-circuited end of the choke, formed and by the physical contact of end region 43 of conductor portion 41a and conductor portion 41b, is filled with a cos cos nor (62; Sm Sm n02: second material having a dielectric constant where v (l3) 6 is greater than 6,. 5

Depending upon the particular frequency discriminaln either case, the two equations are solved simultation desired, the dielectric constants s and s and the neously, as explained hereinabove to determine 6 6 6, relative lengths 0, and 6 of the two regions of the and 6 choke are proportioned as explained hereinabove. In all the embodiments described it was assumed that Fl( vt5, now to be considered, shows a prior art open- 10 all of the input impedance is provided by a particularly elreulted Choke eompnsmg a length of uniform transloaded length of transmission line and its termination.

mission line O length 9, p circuited at one end ln specific instances, however, the input impedance The lhPut Impedance m at the other end 52 of such a can also include a lumped reactance at the input end line is given y of the choke which results from the nature of the struc- Zm izocot 0, l5 ture For example, in FIG. 4 there is a discontinuity at the input end of the choke m the region of gap 46. (8) Since the resulting lumped capacitive reactance will where Z is the characteristic-impedance of the line. mqdlfy the rfisults obtalned by 0 the relemm At those frequencies of which 0 is equal to a quarter 0 Palrs of equatlohs as Outhheq herelhabove; h) l p f a wavelength, or odd multiples th f Zm is equal reactances should be taken into accountif significant. to zero. At those frequencies for which 0 is equal to half It gnderstqod that the above described firrahge a wavelength or mumphas th f approaches ment 1S illustrative of but one of the many possible spefinity. To modify these harmonic relationships, the clfie ehlbedlmehts whleh h represent appheatlehs of prior art open-circuit choke is modified, in accordance the prlhelples of the lhvehtloh- Thus, numerous and i h h prcsam invention, b uncquzn l di h varied other embodiments can readily be devised in actransmission live in the manner illustrated in FIG. 7 eerdahee with he Principles y those Skilled in the which shows a length of transmission line 60 open cir- Without p ng from the pirit and 560136 Of the cuitcd at the far end 61'. A first region of said line, adjainvention. cent to the input end 62 is loaded by means of a first hat iS Claim d i5 dielectric constant 6,. A second region, constituting the l. A dielectric-loaded choke comprising: balance of said line, is loaded by means ofa second di- :1 length of transmission line open circuited at its electric material having a dielectric constant e where input end and reactively terminated at its other e, is larger than 6 The normalized input impedance for end; 4 such a choke is given by a first region of said line, adjacent to the input end,

in F cos n6 sin n6 sin n9 cos n9 For the case ofa low input impedance at a fundamenbeing loaded by means of a first dielectric material tal frequency (Z,-,, 0 for n 1) and a high impedance having a dielectric constant e at 1111 Odd hllfmOhie m for We g the a second region of said line, constituting the remainmerator of equation (9) equal to zero for the fundaing portion of said line, being loaded by means of mental, and the denominator equal to ero o e ara second dielectric material having a second dielecmonic, thus obtaining the following relationships tric constant 6 where 5 and e, are unequal;

the arameters ofsaid choke, includin the-len ths of COS CO5 62- (EJQ) Sm Sm 02- 0 said two regions, and the magnitudgs of 6, End 6 being proportioned to p'roduce'an input impedance and to said choke that is much different at one frequency than it is at a second frequency wheresaid V5005 1 Sin 2+ a Sih 1 C05 1192: two frequencies are harmonically related.

(H) 2. The choke according to claim 1 wherein said reactive termination is a short circuit. In a choke which exhibits a high impedance at a 3. The choke according to claim 1 wherein said reacd m frequency Z O at n 1 d a much two termination is an open circuit. lower impedance at some h i frequency Z =0 4. The choke according to claim I wherein: at 1), we set h denominator f equation 9) equal said reactive termination is a short circuit; to zero for the fundamental, and the numerator equal 2 is greater than to zero for the harmonic, thus obtaining a second pair and wherein the Parameters of Said Choke are p f equations portioned such that the input impedance of said choke at said one frequency is much larger than the 5 COS 0] Sin 02+ 0] CO5 02 0 input impedance-at said second frequency, where said second frequency is an odd harmonic of said (12) one frequency.

7 5. The choke according to claim 4 wherein the parameters of said choke are related by cos 0, cos \/(e 7e sin 6, sin 0 (l and l/ VeTcos 6, sin n6 l/ VeTsin n0, cos n0 0;

l/ cos 0 sin 0 l/ Velsin 6, cos 0 =0 and cos n0 cos n0 V(e /e sin n0 sin n0 0;

where 0 and 0 are the electrical lengths respectively, of said first and second regions at said one frequency;

and n is an integer defining the harmonic order of said second frequency.

8. The choke according to claim 1 wherein said transmission line is a conductively bounded waveguide;

and wherein said dielectric materials fill the bounded volume.

I 8 9. The choke according to claim 1 wherein said transmission line comprises an annular recess formed in the inner conductor of'a coaxial cable.

10. The choke according to claim 1 wherein: 5 said reactive termination is an open circuit:

e, is greater than 6 and wherein the parameters of said choke are proportioned such that the input impedance of said choke at said one frequency is much less than the input impedance at said second frequency, where said second frequency is an odd harmonic of said one frequency. 11. The choke according to claim 10 wherein the parameters of said choke are related by Vejcos 6 sin 0 VeYsin 0, cos 6 0.

and

cos n0 cos n0 V2/| sin n0 sin n0 0;

where 0 and 0 are the electrical lengths, respectively, of said first and second regions at said one frequency;

and n is an integer defining the harmonic order of 

1. A dielectric-loaded choke comprising: a length of transmission line open circuited at its input end and reactively terminated at its other end; a first region of said line, adjacent to the input end, being loaded by means of a first dielectric material having a dielectric constant Epsilon 1; a second region of said line, constituting the remaining portion of said line, being loaded by means of a second dielectric material having a second dielectric constant Epsilon 2, where Epsilon 2 and Epsilon 1 are unequal; the parameters of said choke, including the lengths of said two regions, and the magnitudes of Epsilon 1 and Epsilon 2, being proportioned to produce an input impedance to said choke that is much different at one frequency than it is at a second frequency where said two frequencies are harmonically related.
 2. The choke according to claim 1 wherein said reactive termination is a short circuit.
 3. The choke according to claim 1 wherein said reactive termination is an open circuit.
 4. The choke according to claim 1 wherein: said reactive termination is a short circuit; epsilon 2 is greater than epsilon 1; and wherein the parameters of said choke are proportioned such that the input impedance of said choke at said one frequency is much larger than the input impedance at said second frequency, where said second frequency is an odd harmonic of said one frequency.
 5. The choke according to claim 4 wherein the parameters of said choke are related by cos theta 1 cos theta 2- Square Root ( epsilon 1/ epsilon 2) sin theta 1 sin theta 2 0 and 1/ Square Root epsilon 2 cos theta 1 sin n theta 2 1/ Square Root epsilon 1 sin n theta 1 cos n theta 2 0; where theta 1 and theta 2 are the electrical lengths, respectively of said first and second regions at said one frequency; and n is an odd integer defining the odd harmonic order of said second frequency.
 6. The choke according to claim 1 wherein the parameters of said choke are proportioned sucH that the input impedance of said choke at said one frequency is much lower than the input impedance of said choke said second frequency.
 7. The choke according to claim 6 wherein the parameters of said choke are related by 1/ Square Root epsilon 2 cos theta 1 sin theta 2 + 1/ Square Root epsilon 2 sin theta 1 cos theta 2 0 and cos n theta 1 cos n theta 2- Square Root ( epsilon 1/ epsilon 2) sin n theta 1 sin n theta 2 0; where theta 1 and theta 2 are the electrical lengths respectively, of said first and second regions at said one frequency; and n is an integer defining the harmonic order of said second frequency.
 8. The choke according to claim 1 wherein said transmission line is a conductively bounded waveguide; and wherein said dielectric materials fill the bounded volume.
 9. The choke according to claim 1 wherein said transmission line comprises an annular recess formed in the inner conductor of a coaxial cable.
 10. The choke according to claim 1 wherein: said reactive termination is an open circuit; epsilon 1 is greater than epsilon 2; and wherein the parameters of said choke are proportioned such that the input impedance of said choke at said one frequency is much less than the input impedance at said second frequency, where said second frequency is an odd harmonic of said one frequency.
 11. The choke according to claim 10 wherein the parameters of said choke are related by cos theta 1 cos theta 2- Square Root ( epsilon 2/ epsilon 1) sin epsilon 1 sin epsilon 2 0 and <--> epsilon 2 cos n theta 1 sin n theta 1+ Square Root epsilon 1 sin n theta 1 cos n theta 2 0; where theta 1 and theta 2 are the electrical lengths, respectively, of said first and second regions at said one frequency; and n is an odd integer defining the odd harmonic frequency of said second frequency.
 12. The choke according to claim 10 wherein the parameters of said choke are related by Square Root epsilon 2 cos theta 1 sin theta 2+ Square Root epsilon 1 sin theta 1 cos theta 2 0 and cos n theta 1 cos n theta 2- Square Root ( epsilon 2/ epsilon 1 sin n theta 1 sin n theta 2 0; where theta 1 and theta 2 are the electrical lengths, respectively, of said first and second regions at said one frequency; and n is an integer defining the harmonic order of said second frequency. 